A Global Navigation Satellite System (GNSS) transmits ranging and other signals, which are used by land, sea, and air users to determine a three-dimensional position, a velocity and a time of day. The GNSS includes a network of satellites to transmit the signals to users. One example of a GNSS is the Global Positioning System (GPS). GPS includes a network of 24 or more satellites, called GPS satellites, which operate in medium earth orbit.
The position solutions that GNSS users determine are inaccurate to some extent, due to factors such as errors in satellite ephemeris data, satellite clock drift, ionosphere and troposphere delays, multi-path signals, and receiver noise. In order to determine how large the inaccuracies in their position solutions could be, GNSS users may compute integrity bounds, which are high-assurance bounds for the maximum possible distance errors in their position solutions. Integrity bounds constrain the amount of uncertainty in the position solutions determined by GNSS users. Because the probability is exceedingly small that the errors in a GNSS user's position solution could be greater than the integrity bounds, users are virtually guaranteed that their actual positions are within the integrity-bounded distances of the position solutions that they determine using the GNSS. In general, a navigation system with more integrity allows users to compute smaller integrity bounds, which more closely bound the possible errors, and thus reduce the uncertainty in users' determined positions. The level of integrity provided by a GNSS itself may be insufficient for users to determine integrity bounds that are sufficiently small to meet the requirements for some applications, such as aircraft navigation.
Other systems called augmentation systems generate correction and integrity data, and broadcast the data to users. Users employ the data from the augmentation systems to supplement the data that they receive from the GNSS, in order to increase the accuracy and integrity of their position solutions. The correction data generated by the augmentation system are adjustments that compensate for some of the errors introduced by factors such as those listed above. Users employ the correction data in order to increase the accuracy of their position solutions. The integrity data include values that bound the residual errors that may still remain even after correction data are applied. Users employ the integrity data in order to determine tighter integrity bounds, which reduce the extent of possible inaccuracies remaining in their position solutions.
A typical augmentation system for use with GPS includes one or more reference stations with GPS receivers that receive GPS data from the GPS satellites. The precise locations of the reference station receivers are known. A master control station receives the GPS data collected by the reference stations and determines for each GPS satellite the bias between the expected range to the satellite, based on the known locations of the reference station receivers, and the observed range to the satellite, determined using the GPS data. The master control station also monitors the performance of the GPS satellites and reports anomalies to users, thereby providing increased integrity for GPS users. The master control station generates correction and integrity data that are broadcast to users.
One known type of augmentation system is the satellite-based augmentation system (SBAS). A typical SBAS includes a set of reference stations with GPS receivers at locations throughout the wide geographic area serviced by the SBAS. The correction and integrity data generated by the SBAS master control station are transmitted to one or more geosynchronous earth orbit (GEO) satellites for broadcast to users throughout the wide area on the same frequency and in a similar format to GPS satellites. This method allows signals from the SBAS GEO and the GPS satellites both to be received by the user's GPS receiver, and has the added benefit of providing the SBAS GEO satellites as additional ranging sources to improve availability. Because the SBAS service area is limited by the footprints of the SBAS GEO satellites (which span approximately +/−76 degrees in longitude and latitude) and the quantity and locations of the SBAS reference stations distributed throughout the service area, a single SBAS may provide service to users throughout an entire nation or continent.
Another known type of augmentation system is the ground-based augmentation system (GBAS). A typical GBAS includes a set of reference stations with GPS receivers located within the local area (e.g., 20 or 30 mile radius) serviced by the GBAS. The GBAS master control station generates correction and integrity data, which are broadcast to users in the immediate vicinity of the GBAS via a VHF data link. Because the GBAS service area is limited by the coverage area of the VHF transmitter and by the localized distribution of its reference stations, a single GBAS may be used only within a local area, such as around an airport. Since the GBAS operates in a local area only, it may provide more accurate correction data and more tightly bounded integrity data for that area, and it may alert users to fault conditions more quickly than an SBAS that provides service in the same area.
A further known type of augmentation system, which does not use reference stations at fixed locations, is the air-based augmentation system (ABAS). The ABAS typically uses only the user's own GPS receiver to receive the GPS data from GPS satellites, and it may also supplement the GPS data with data from other equipment, such as an inertial navigation system (INS). The ABAS uses the Receiver Autonomous Integrity Monitoring (RAIM) method to perform integrity monitoring. Since RAIM is based on comparing the results achieved using different combinations of GPS satellites, the ABAS requires more GPS satellites in view than the minimum four required for basic position fixing. Hence, the ABAS gains integrity at the expense of possibly reduced availability.
Authorization for using a GNSS for aircraft navigation is granted by Government and international organizations, which define requirements for different GNSS navigation services that may be used for different types of aircraft navigation (e.g., for different phases of flight). Some GNSS navigation services are supported by the GNSS alone, others are supported by the GNSS supplemented with an augmentation system, such as an SBAS or GBAS. The requirements for each type of GNSS navigation service specify limit thresholds for integrity bounds, which constrain the maximum errors in user position solutions that are allowable when using that type of navigation service. A navigation service is available for a user (i.e., it is approved for use) when the user is able to determine that the integrity bounds are not greater than the threshold limit defined for that service. For example, the requirements for the GNSS navigation service for Category 1 aircraft landings specify that the integrity bound for a user's horizontal position solution be no greater than 40 meters, and the integrity bound for the user's vertical position solution be no greater than 10 meters.
The magnitudes of the integrity bounds determined by a user are dependent on several factors, including the locations of GNSS satellites relative to the user, the current performance of the GNSS and the augmentation system (if applicable), and environmental conditions (e.g., troposphere and ionosphere). These factors typically change over time and vary by user location. It is typical for the integrity bounds determined by the same user at different times to vary, even if the user remains at the same location. It is also typical for the integrity bounds determined at the same time by users at different locations to be different. At any time, a GNSS navigation service may be available to users at some locations (those whose integrity bounds are within the required thresholds), but unavailable to users at other locations (those whose integrity bound exceed the required thresholds).